Acoustic printing is a potentially important direct marking technology. It still is in an early stage of development, but the available evidence indicates that it is likely to compare favorably with conventional ink jet systems for printing either on plain paper or on specialized recording media, while providing significant advantages of its own.
More particularly, acoustic printing does not require the use of nozzles with small ejection orifices which easily clog. Therefore, it not only has greater intrinsic reliability than ordinary drop on demand and continuous stream ink jet printing, but also is compatible with a wider variety of inks, including inks which have relatively high viscosities and inks which contain pigments and other particulate components. Furthermore, it has been found that acoustic printing provides relatively precise positioning of the individual printed picture elements ("pixels"), while permitting the size of those pixels to be adjusted during operation, either by controlling the size of the individual droplets of ink that are ejected or by regulating the number of ink droplets that are used to form the individual pixels of the printed image. See a copending and commonly assigned United States patent application of Elrod et al, which was filed Dec. 19, 1986 under Ser. No. 944,286 on "Variable Spot Size Acoustic Printing".
As is known, an acoustic beam exerts a radiation pressure against objects upon which it impinges. Thus, when an acoustic beam impinges on a free surface (i.e., liquid/air interface) of a pool of liquid from beneath, the radiation pressure which it exerts against the surface of the pool may reach a sufficiently high level to release individual droplets of liquid from the pool, despite the restraining force of surface tension. Focusing the beam on or near the surface of the pool intensifies the radiation pressure it exerts for a given amount of input power. These principles have been applied to prior ink jet and acoustic printing proposals. For example, K. A. Krause, "Focusing Ink Jet Head," IBM Technical Disclosure Bulletin, Vol 16, No. 4, September 1973, pp. 1168-1170 described an ink jet in which an acoustic beam emanating from a concave surface and confined by a conical aperture was used to propel ink droplets out through a small ejection orifice. Lovelady et al. U.S. Pat. No. 4,308,547, which issued Dec. 29, 1981 on a "Liquid Droplet Emitter," showed that the small ejection orifice of the conventional ink jet is unnecessary. To that end, they provided spherical piezoelectric shells as transducers for supplying focused acoustic beams to eject droplets of ink from the free surface of a pool of ink. They also proposed acoustic horns driven by planar transducers to eject droplets of ink from an ink coated belt. Moreover, concurrently herewith, to increase the resolution which can be achieved and to provide a less cumbersome and lower cost technique for manufacturing arrays of relatively stable acoustic droplet ejectors, a copending and commonly assigned United States patent application of Elrod et al, which was filed Dec. 19, 1986 under Ser. No. 944,698 on "Acoustic Lens Arrays for Ink Printing" is introducing acoustic lenses for performing the beam focusing function, Also see another copending and commonly assigned United States patent application of Elrod et al, which was filed Dec. 19, 1986 under Ser. No. 944,145 on "Microlenses for Acoustic Printing".
The shell-like piezoelectric transducers and acoustic focusing lenses which have been developed for acoustic printing have concave beam forming surfaces. In practice, these beam forming surfaces typically have an essentially constant radius of curvature, regardless of whether they are spherical or cylindrical, because they are designed to cause the acoustic beams which they launch to come to a sharp focus at or near the free surface of the ink. A diffraction limited focus is the usual design goal for an acoustic lens, while an unaberrated focus is the usual design goal for a shell-like, self-focusing transducer. Unfortunately, however, the concavity of the beam forming surfaces of these devices causes them to collect and retain particulates which precipitate out of the ink or otherwise deposit on them. These deposits can cause unwanted defocusing of the acoustic beams, especially if they are permitted to accumulate over an extended period of time.